This invention relates to a robot control apparatus for controlling an industrial robot having a rotatable-type arm and, more particularly, to a robot control apparatus wherein an excessive force is prevented from being applied to the arm when the arm is rotated.
The rising cost of labor has given rise to a demand for labor-saving measures and for improvement in work procedures. Industrial robots have been put into use to meet this demand and have been very successful in doing so. Particularly outstanding results have been achieved in plants having numerous machine tools where industrial robots are used to perform such simple services as changing the workpieces and tools associated with each machine tool. The demand for such robots has grown yearly. FIGS. 1A and 1B illustrate an industrial robot of the type described, in which FIG. 1A is a plan view and FIG. 1B a side view. FIGS. 1A and 1B, numeral 1 denotes a mechanical hand for gripping a workpiece when, for example, it is to be exchanged, numeral 2 denotes a wrist which is capable of being swiveled (.alpha.-axis) and bent up and down (.beta.-axis), numeral 3 denotes an arm which can be freely extended and retracted (R-axis), and numeral 4 denotes a casing which can be moved vertically (Z-axis) along a shaft PL and rotated (.theta.-axis). Numeral 5 denotes a frame, 6 a teaching box for teaching robot motions, 7 an operator's panel for manipulation by the operator, and 8 a control unit for storing, in succession, the data input through the teaching box 6, such as the operating position (point), operating speed and the various services, and for controlling the motion of the mechanical hand 1, wrist 2, arm 3 and casing 4 in accordance with the data input through the teaching box 6.
In playback type industrial robots, the teaching box 6 is used to define the service operations in advance, and the data input through the teaching box 6 (referred to as robot instruction data hereinafter) is stored in a memory within the control unit 8. Whenever a request for service is received by the robot, a series of robot instruction data is read from the memory sequentially, and the robot responds by servicing the machine.
Extending the arm 3 of the robot increases the moment of the force associated with the axis of rotation (.theta.-axis). Therefore, when the casing 4 of the arm 3 is rotated at high speed while the arm 3 is extended, an excessive force acts upon the drive unit associated with the axis of rotation and upon other units as well and can cause damage to the robot. The maximum speed set forth in a robot's specifications pertains to rotation when the arm 3 is in the fully retracted state. A considerable excessive force acts upon the robot when an attempt to rotate the arm 3 at the maximum speed while the arm 3 is extended and gripping a heavy load occurs. One specific example of robot damage is wear sustained by a reduction mechanism within the rotational axis drive unit when motion about the .theta.-axis is started and stopped.
In order to prevent the arm 3 and drive units from being acted upon by the excessive force, it is conventional practice to provide means for inputting a low-speed command when the casing 4 is to be rotated with the arm 3 extended. However, it is troublesome to find suitable values for the speed command during the teaching operation, and difficulty is encountered in selecting a suitable speed command in a case where movement about the .theta.-axis and along the R-axis occurs simultaneously. In such a case the maximum safe speed is constantly changing.